Wells for the recovery of hydrocarbons or minerals from a subsurface formation are commonly drilled by connecting a drill bit onto the lower end of an assembly of drill pipe sections connected end-to-end (commonly referred to as a “drill string”), and then rotating the drill string so that the drill bit progresses downward into the earth to create the desired wellbore. The drill string is typically rotated by means of a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the wellbore.
The primary components of a typical drilling rig include a base support structure, a drill floor, and a mast (also called a derrick) supported on and extending upward from the drill floor. The drill floor is typically elevated well above the ground surface to provide space to accommodate and allow access to various equipment required for drilling operations. An arrangement of pulleys (sheaves) called a crown block is mounted to the top of the rig mast. Hoisting apparatus called a drawworks, comprising a cable drum, a cable winch, and ancillary equipment, is provided in association with the drill floor. Wire-rope cable is fed from the cable drum up to the crown block and threaded over the various sheaves in the crown block, and then down to a “traveling block”, which is an assembly of sheaves that is free to move vertically within the mast structure as the wire-rope cable (“drill line”) is played out or taken up by the cable winch and drum of the drawworks. The traveling block has a lifting hook to support equipment used to raise and lower the drill string, and to add pipe sections during drill string assembly (or “make-up”) and to remove pipe sections during drill string disassembly (“break-out”). For drilling operations using a top drive instead of a rotary table to rotate the drill string, the top drive is suspended from the traveling block hook.
For optimal efficiency and economy in well-drilling operations, it is desirable for drilling rigs to be readily transportable, rapidly erected, and rapidly disassembled for transportation to new wellsites. Accordingly, the transportability of rig components and the speed at which components can be assembled with the minimum amount of auxiliary equipment are paramount concerns. Conventional types of transportable rigs may require auxiliary support equipment to facilitate the erection and disassembly of large components such as the base, the drill floor, and the pipe racking board, thereby increasing rig set-up, take-down, and operational costs.
Numerous types of transportable rigs may be found in the prior art. Known transportable rigs commonly feature what is called a “bootstrap mast”. A bottom mast section having a large lower opening on one side is mounted to the drill floor, using a mobile crane. The crane then lowers the top section of the finished mast (housing the crown block) into the bottom mast section, and the top mast section is temporarily pinned to the bottom mast section. Next, the crane positions an intermediate mast section through the opening in the bottom mast section so that the upper end of the intermediate mast section can be securely connected to the lower end of the upper mast section. The traveling block is then lowered to engage and support the intermediate section, the upper mast section is unpinned from the bottom mast section, the drawworks is actuated to hoist the upper and intermediate mast sections a distance corresponding to the height of the intermediate mast section, and then the partially-constructed mast assembly (i.e., upper section plus one intermediate section) is temporarily pinned to the stationary bottom mast section. This latter process is then repeated as necessary to install additional intermediate mast sections until the mast has reached its intended final height, whereupon the lowermost intermediate mast section is secured to the bottom mast section so that the rig is ready to be put into service.
Drilling sites are often located in remote areas requiring truck transportation of rig components and equipment required for rig assembly (or “rig-up”). Further complicating the rig-up process is the common need to relocate the rig to a more promising site after a wellbore has been drilled and it has been determined that the wellbore will not be sufficiently productive to warrant completion and operation of the well. Wellsite changes can occur once every several months, and, in response, prior art rig systems have attempted to increase the mobility of rig components and the efficiency of rig erection and disassembly procedures in order to minimize associated costs. However, the need for auxiliary equipment typically remains necessary for performing steps such as placing the drill floor.
Since the variable costs associated with leased support equipment, such as cranes and the like, are calculated on a per-hour or per-day basis, expediting rig take-down, transport, and set-up operations is crucial for minimizing equipment leasing costs. Typically, rig take-down and set-up times are in the order of days, and very large equipment and labor costs can be incurred for each end of a set-up and take-down operation. Various prior art drilling rigs are geared towards facilitating rapid set-up, take-down and transport, but they still require auxiliary equipment such as external cranes and external winches, which most often need to be leased and therefore increase overall rig set-up and take-down costs, particularly for remote wellsites.
One approach to reducing rig set-up times and costs is to raise the drill floor to its intended service elevation after erection of the mast. This results in economies due to the fact that the mast can be erected with the drill floor close to ground level rather than in its final position 20 feet or more above ground. The cranes and other equipment manipulating and positioning the various drill floor and mast components do not require as high a reach, and workers have more ready access to the drill floor during rig-up procedures.
An example of a transportable drilling rig having a drill floor that can be raised after mast erection can be seen in U.S. patent application Ser. No. 12/492,980 (Wasterval), Pub. No. US 2012/0326734A1. Wasterval teaches the construction of a drill floor over a base structure, which is provided with hydraulic cylinders for raising the drill floor above the base structure. After the drill floor has been raised by an increment corresponding to the stroke of the hydraulic cylinders, a first set of box beams are disposed between the drill floor and the base structure, and the box beams are anchored to the drill floor. The lift cylinders on the base structure can then be retracted to engage lift points on the box beams. The lift cylinders are then actuated again, this time to raise both the drill floor and the first box beams anchored thereto. This allows insertion of a second set of box beams between the base structure and the first set of box beams, thus elevating the drill floor a further incremental amount. This procedure is repeated as necessary to install additional sets of box beams until the drill floor has reached its intended elevation.
The Wasterval system thus allows the drill floor to be erected or constructed close to ground level, followed by erection of the rig mast on the drill floor, whereupon the drill floor (with erected mast) can be elevated as required. However, this system has an inherent drawback in that the rig floor raising procedure has to be carried out in incremental and comparatively complex stages, and is correspondingly complex and time-consuming. In addition, the Wasterval system entails the provision of a robust hydraulic system, which might not otherwise be needed on site during rig-up, and thus increases rig-up costs. As well, the Wasterval system requires the use of mobile cranes or other auxiliary hoisting equipment to manipulate and position the box beams, further adding to rig-up costs.
For the foregoing reasons, there is a need for an improved transportable drilling rig that can be rapidly erected and disassembled with minimal need for auxiliary equipment. In particular, there is a need for an improved transportable rig in which the drill floor can be elevated after erection of the mast, but without requiring hydraulics or auxiliary hoisting equipment.